The present invention relates to trigger shifters for bicycles and more particularly to a release mechanism for a trigger shifter for a bicycle.
German Utility Model DE 90 15 515 discloses a shifter for controlling a bicycle gearing system that includes a lever and a pair of pawls. The pawls engage a toothed disk that is connected to a cable spool for winding and unwinding the shifting cable. Upon actuation of the lever in the tension direction or the release direction, the respective gear is engaged and the correspondingly associated shifting travel is achieved by winding or unwinding the shifting cable on the cable spool. Once the actuation has occurred the lever returns to its initial position. Friction devices or catch devices are arranged in the shifter housing and cooperate with the cable spool or the toothed disk to retain the cable spool in the selected position. The pawls have a configuration such that when one pawl snaps into engagement with the toothed disk, the other pawl is disengaged from the toothed disk. Since the cable spool is maintained in its current position by a spring-loaded catch device which only remains effective up to a certain retention force, the cable spool may rotate unexpectedly because it is pretensioned by the shifting cable. When the lever is actuated in the release direction, the retention force of the catch device is overcome. The cable spool, pretensioned by the shifting cable, is thereby rotated on to the next retaining position and the corresponding shifting travel is enabled. Because of the nonpositive catch device, it occasionally happens that the intended gear position is overshot during shifting operations in the release direction. There is a need for a more reliable shifter that maintains the desired gear ratio.